Thin steel sheet and process for producing the same

ABSTRACT

A thin steel sheet having sheet thickness ≦1.6 mm, but tensile strength ≧780 MPa and Young&#39;s modulus ≧240 GPa in transverse direction is provided, where the steel sheet has composition including, in mass %, C: 0.06-0.12%, Si: 0.5-1.5%, Mn: 1.0-3.0%, P: 0.05% or less, S: 0.01% or less, Al: 0.5% or less, N: 0.01% or less, Ti: 0.02-0.20%, and the balance being Fe and incidental impurities, where the composition satisfies relations of Formula (1) and (2), and microstructure such that ferrite phase has area ratio ≧60% and martensite phase has area ratio of 15-35%, ferrite and martensite phases are 95% or more in total, average grain size of ferrite is ≦4.0 μm and that of martensite is ≦1.5 μm, 
         0.05≦[ % C]−( 12/47.9 )×[% Ti*]≦ 0.10    (1),
 
       where 
       Ti*=[% Ti]−( 47.9/14 )×[% N]−( 47.9/32.1 )×[% S]  (2).

TECHNICAL FIELD

This disclosure relates to a high-strength thin steel sheet havingexcellent rigidity that is preferably and mainly used for automobilebody parts, and a method for manufacturing the same. The high-strengththin steel sheet, which is preferably applicable as structural membershaving a columnar or nearly columnar cross-sectional shape with a rigidsensitivity index of sheet thickness of approximately 1, such as centerpillars, side sills, side frames and cross members of automobiles, has atensile strength of 780 MPa or higher and exhibits excellent ductility.

BACKGROUND

In recent years, responding to increasing public concern about globalenvironmental issues, for example, emission regulations have beenimplemented for automobiles and it has been a critical issue to reducethe weight of automobile bodies. As such, efforts have been made toreduce the weight of such bodies by strengthening steel sheets to reducethe sheet thickness. Currently, as a result of remarkable advances instrengthening steel sheets, there is increased use of steel sheetshaving a sheet thickness of less than 1.6 mm. Particularly, since steelsheets in 780 MPa and 980 MPa grades of tensile strength have been usedin increasing proportions every year, it is essential to prevent adecrease in rigidity of parts due to reduced thickness at the same timeto achieve such weight reduction through strengthening of steel sheets.The problem associated with a decrease in rigidity of parts due toreduced thickness of steel sheets becomes apparent in those steel sheetshaving a tensile strength of 590 MPa or higher.

Generally, to increase the rigidity of parts, it is thought to beeffective to change the shape of those parts or, alternatively, forthose parts being subjected to spot welding, to change the weldingconditions such as increasing the number of welding points or switchingto laser welding.

However, when used as automobile parts, there is a problem that it isnot easy to change the shape of the parts in a limited space in anautomobile, and changes to the welding conditions are made at theexpense of an increase in cost, and so on.

In view of the foregoing, to increase the rigidity of parts withoutchanging the shape or welding conditions of the parts, it is effectiveto increase the Young's modulus of members used for these parts. In thecase of steel having a body-centered cubic lattice, it is known that theYoung's modulus, which is strongly dependent on texture, has the highestvalue in the <111> direction in which atoms are most densely packed,while having the smallest value in the <100> direction in which atomsare less dense. It is widely known that the Young's modulus of normaliron which is less anisotropic in crystal orientation is approximately210 GPa. However, if the crystal orientation is anisotropic and theatomic density can be increased in a particular direction, the Young'smodulus can be increased in that direction.

Conventionally, as for the Young's modulus of steel sheets, variousconsiderations have been given to increasing Young's modulus in aparticular direction by controlling the texture.

For example, JP 5-255804 A discloses a technique that uses steelresulting from adding Nb or Ti to ultra low carbon steel and involvescontrolling, in a hot rolling step, the rolling reduction ratio to be85% or more in a temperature range of Ar3 to (Ar3+150° C.) and therebyfacilitating transformation of non-recrystallized austenite to ferrite,so that ferrite in {311}<011> and {332}<113> orientations is allowed togrow at the stage of hot-rolled sheet, and the Young's modulus isincreased in a direction perpendicular to the rolling direction throughthe subsequent cold rolling and recrystallization annealing whereby{211}<011> orientation is made into the primary orientation.

In addition, JP 8-311541 A discloses a method of manufacturing ahot-rolled steel sheet with an increased Young's modulus by adding Nb,Mo and B to low carbon steel having C content of 0.02% to 0.15% andcontrolling the rolling reduction ratio to be 50% or more in atemperature range of Ar3 to 950° C., thereby causing growth in{211}<011> orientation.

Further, JP 2006-183131 A and JP 2005-314792 A disclose techniques thatuse steel resulting from adding Nb to low carbon steel, define thecontent of C that is not fixed as carbonitride, and involve controlling,in a hot rolling step, the total rolling reduction ratio to be 30% ormore at 950° C. or lower to facilitate transformation ofnon-recrystallized austenite to ferrite so that ferrite in {113}<110>orientation is allowed to grow at the stage of hot-rolled sheet and theYoung's modulus is increased in a direction perpendicular to the rollingdirection through the subsequent cold rolling and recrystallizationannealing whereby the {112}<110> orientation is made into the primaryorientation.

However, the above-mentioned conventional techniques have the followingproblems.

That is, while the technique disclosed in JP 5-255804 A increases theYoung's modulus of a steel sheet by using ultra low carbon steel havinga C content of 0.01% or less and controlling its texture, the obtainedtensile strength is about 450 MPa at most. Thus, there was a limit tofurther strengthening by applying this technique.

The technique disclosed in JP 8-311541 A has a problem that it cannotutilize texture control by cold working since the target steel sheet isa hot-rolled steel sheet, where it is difficult not only to achieve evena higher Young's modulus, but also to manufacture such a high strengthsteel sheet that has a sheet thickness of less than 2.0 mm in a stablemanner by low temperature finish rolling.

While the technique disclosed in 2006-183131 A increases tensilestrength by increasing the amount of alloying elements to be added andincreasing the fraction of martensite, it was difficult to improveworkability while enhancing the strength, because total elongation isdecreased and strength-elongation balance (TS×El) is distorted as well.

In addition, while the techniques disclosed in 2006-183131 A and JP2005-314792 A increase the Young's modulus by controlling the totalrolling reduction ratio at 950° C. or lower to be 30% or more in the hotrolling step, these techniques suffered a problem that it was difficultto maintain a total rolling reduction ratio of 30% or more due to highrolling load in a temperature range of 950° C. or lower.

As such, the conventional techniques are directed to increasing theYoung's modulus of steel sheets such as hot-rolled steel sheets or mildsteel sheets, having a large sheet thickness, materials having highstrength but poor ductility, or materials difficult to produce. Thus, itwas difficult to provide a high strength steel sheet, which has a sheetthickness of 1.6 mm or less and TS of 780 MPa or higher, with bothhigher ductility and higher Young's modulus by using such conventionaltechniques.

It could therefore be helpful to provide a high-strength thin steelsheet having excellent rigidity that has a sheet thickness as small as1.6 mm or less, but a tensile strength as high as 780 MPa or higher,more preferably 980 MPa or higher, in a transverse directionperpendicular to the rolling direction (hereinafter, also referred to asthe “transverse direction”), and satisfies a condition that the Young'smodulus in the transverse direction is 240 GPa or higher, as well as anadvantageous method for manufacturing the same.

SUMMARY

In the case of normal steel having a body-centered cubic lattice, theYoung's modulus of the steel, which is largely dependent on texture, ishigh in the <111> direction in which atoms are most densely packed,while being low in the <100> direction in which atoms are less dense.Accordingly, growth in the (112)[1-10] orientation brings aboutalignment of the <111> direction with the transverse direction of thesteel sheet. It is thus possible to increase the Young's modulus in thisdirection.

In addition, there are various methods for strengthening steel. Forexample, Dual-Phase (DP) steel in which a soft ferrite phase isstrengthened with a hard martensite phase, is known to have generallygood ductility. However, in ultra high strength steel having TS of 780MPa or higher, the volume fraction of martensite phase tends to increasein general, which results in not only deterioration in ductility, butalso difficulty in causing growth in the (112)[1-10] orientation, whichis effective in increasing the Young's modulus in the transversedirection.

To solve the above-described problem, as a result of studies on theYoung's modulus of a high-strength thin steel sheet having TS of 780 MPaor higher in a direction perpendicular to the rolling direction, wefound that it is possible to keep the volume fraction of martensite loweven in a steel sheet having an ultra high strength of TS of 780 MPa orhigher by solid solution strengthening, grain refinement strengtheningand precipitation strengthening, and to balance high ductility, highstrength and high rigidity by increasing the accumulation of ferrite inthe (112)[1-10] orientation.

We thus provide:

[1] A thin steel sheet having a composition including, in mass %, C:0.06% to 0.12%, Si: 0.5% to 1.5%, Mn: 1.0% to 3.0%, P: 0.05% or less, S:0.01% or less, Al: 0.5% or less, N: 0.01% or less, Ti: 0.02% to 0.20%,and the balance being Fe and incidental impurities, where thecomposition satisfies relations of Formula (1) and (2) below,

wherein the steel sheet has a microstructure such that a ferrite phasehas an area ratio of 60% or more and a martensite phase has an arearatio of 15% to 35%, where a total of the ferrite phase and themartensite phase is 95% or more, an average grain size of ferrite is 4.0μm or less and an average grain size of martensite is 1.5 μm or less,

wherein the steel sheet has a tensile strength (TS) of 780 MPa or higherin a transverse direction perpendicular to the rolling direction,Young's modulus of 240 GPa or higher in the transverse direction, andstrength-elongation balance (TS×El) of 16500 MPa·% or more in thetransverse direction, the strength-elongation balance being expressed bya product of the tensile strength (TS) and total elongation (El),

0.05≦[% C]−(12/47.9)×[% Ti*]≦0.10   (1),

where

Ti*=[% Ti]−(47.9/14)×[% N]−(47.9/32.1)×[% S]  (2), and

[% M] indicates the content (mass %) of M element.

[2] The thin steel sheet according to item [1] above, wherein thecomposition of the steel sheet further includes, in mass %, Nb: 0.02% to0.10%, and satisfies a relation of Formula (3) below in place of theFormula (1):

0.05 [% C]−(12/92.9)×[% Nb]−(12/47.9)×[% Ti*]23 0.10   (3).

[3] The thin steel sheet according to item [1] or [2] above, wherein thecomposition of the steel sheet further includes, in mass %, one or moreelements selected from Cr: 0.1% to 1.0%, Ni: 0.1% to 1.0%, Mo: 0.1% to1.0%, Cu: 0.1% to 2.0% and B: 0.0005% to 0.0030%.

[4] A method for manufacturing a thin steel sheet, the methodcomprising:

in a hot rolling process, subjecting a steel material to finish rollingand completing the finish rolling at 850° C. to 950° C. to obtain ahot-rolled steel sheet, the steel material having a compositionincluding, in mass %, C: 0.06% to 0.12%, Si: 0.5% to 1.5%, Mn: 1.0% to3.0%, P: 0.05% or less, S: 0.01% or less, Al: 0.5% or less, N: 0.01% orless, Ti: 0.02% to 0.20%, and the balance being Fe and incidentalimpurities, where the contents of C, N, S and Ti satisfy relations ofFormula (1) and (2) below;

then coiling the steel sheet at 650° C. or lower;

subjecting the steel sheet to pickling; and

then subjecting the steel sheet to cold rolling at a rolling reductionratio of 60% or more;

in a subsequent annealing process, heating the steel sheet to a soakingtemperature of 780° C. to 880° C. at an average heating rate from(Ac1-100° C.) to Ac1 of 15° C./s or higher;

holding the steel sheet at the soaking temperature for 150 seconds orless; and

cooling the steel sheet to 350° C. or lower at an average cooling rateuntil at least 350° C. of 5° C./s to 50° C./s,

0.05≦[% C]−(12/47.9)×[% Ti*]≦0.10   (1),

where

Ti*=[% Ti]−(47.9/14)×[% N]−(47.9/32.1)×[% S]  (2), and

[% M] indicates the content (mass %) of M element.

[5] The method for manufacturing a thin steel sheet according to item[4] above, wherein the composition of the steel material furtherincludes, in mass %, Nb: 0.02% to 0.10%, and satisfies a relation ofFormula (3) below in place of the Formula (1):

0.05 [% C]−(12/92.9)×[% Nb]−(12/47.9)×[% Ti*]23 0.10   (3).

[6] The method for manufacturing a thin steel sheet according to item[4] or [5] above, wherein the composition of the steel material furtherincludes, in mass %, one or more elements selected from Cr: 0.1% to1.0%, Ni: 0.1% to 1.0%, Mo: 0.1% to 1.0%, Cu: 0.1% to 2.0% and B:0.0005% to 0.0030%.

A high-strength thin steel sheet may be obtained that satisfies theconditions of a tensile strength of 780 MPa or higher, more preferably980 MPa or higher, in the transverse direction and a Young's modulus of240 GPa or higher, more preferably 245 GPa or higher, in the transversedirection and, furthermore, TS×El=16500 MPa·% or more in the transversedirection.

DETAILED DESCRIPTION

Our steel sheets and methods will be specifically described below.

First, the reason why the chemical composition of the steel sheets islimited to the above-described range will be described below.

In addition, although the unit of content of each element included inthe chemical composition of the steel sheet is “mass %,” it will besimply expressed by “%,” unless otherwise specified.

0.06%≦C≦0.12%

C is an element that stabilizes austenite and may improve quenchhardenability and greatly facilitate formation of a low temperaturetransformation phase during a cooling step at the time of annealingafter cold rolling, thereby making a significant contribution toenhancement of strength. To obtain this effect, the C content needs tobe 0.06% or more, more preferably 0.08% or more. On the other hand, a Ccontent exceeding 0.12% leads to an increased volume fraction of a hard,low temperature transformation phase, which results in not only anexcessive increase in strength of steel, but also a deterioration inworkability. In addition, such a high C content inhibitsrecrystallization in an orientation in which the Young's modulus isadvantageously improved in an annealing process after cold rolling.Further, such a high C content also leads to a deterioration inweldability. Thus, the C content should be not more than 0.12%.

0.5%≦Si≦1.5%

Si is one of the important elements. Since Si raises the Ar3transformation point in hot rolling, it facilitates recrystallization ofworked austenite when rolling is performed at a temperature immediatelyabove Ar3. Thus, if the Si content is excessively high exceeding 1.5%, acrystal orientation necessary to increase the Young's modulus can nolonger be obtained. Moreover, addition of a large amount of Si not onlydeteriorates weldability of a steel sheet, but also advances formationof fayalite on a surface of a slab during heating in a hot rollingprocess, thereby facilitating the occurrence of a surface pattern, whichis referred to as so-called “red scales.” Further, when a steel sheet isused as a cold-rolled steel sheet, oxides of Si generated on a surfaceof the steel sheet deteriorates chemical convertibility or,alternatively, when a steel sheet is used as a hot-dip galvanized steelsheet, oxides of Si generated on a surface of the steel sheet inducesabsence of a zinc coating. Thus, Si content should be not more than1.5%. In addition, in the case of a steel sheet requiring surfacetexture or a hot-dip galvanized steel sheet, the Si content ispreferably 1.2% or less.

On the other hand, Si is an element that stabilizes ferrite and is ableto stabilize austenite and facilitate formation of a low temperaturetransformation phase by facilitating transformation to ferrite andconcentrating C in austenite during a cooling step subsequent to soakingin two-phase region in an annealing process after cold rolling. Further,Si may enhance the strength of steel by solid solution strengthening. Toobtain this effect, the Si content should be 0.5% or more, preferably0.7% or more.

1.0%≦Mn≦3.0%

Mn is also one of the important elements. Mn is an austenite-stabilizingelement that may, during a heating step in an annealing process aftercold rolling, lower the Ac1 transformation point, facilitatetransformation of non-recrystallized ferrite to austenite, and allow alow temperature transformation phase formed during a cooling step aftersoaking to grow in an orientation in which the Young's modulus isadvantageously improved, thereby inhibiting a decrease in the Young'smodulus associated with formation of the low temperature transformationphase.

Mn may also improve quench hardenability and greatly facilitateformation of a low temperature transformation phase during a coolingstep after soaking annealing in an annealing process, thereby making asignificant contribution to enhancement of strength. Further, Mn acts asa solid-solution-strengthening element, which also contributes toenhancement of strength of steel. To obtain this effect, the Mn contentshould be 1.0% or more.

On the other hand, a high Mn content exceeding 3.0% severely inhibitsformation of ferrite during cooling after annealing, and even a higherMn content would also deteriorate weldability of the steel sheet. Thus,the Mn content is 3.0% or less, more preferably 2.5% or less.

P≦0.05%

P is an element that segregates at grain boundaries, which results in adeterioration in not only ductility and toughness, but also inweldability of a steel sheet. In addition, P causes an inconveniencethat alloying is delayed when the steel sheet is used as a hot-dipgalvannealed steel sheet. Thus, P content is to be 0.05% or less.

S≦0.01%

S is an element that significantly reduces ductility in hot rolling toinduce hot cracking, and severely deteriorates surface texture. Inaddition, it is desirable to minimize the S content because Sdeteriorates ductility and hole expansion formability by forming coarseMnS as an impurity element. These problems become more pronounced when Scontent exceeds 0.01%. Thus, the S content is 0.01% or less. From theviewpoint of improvement of particularly hole expansion formability, theS content is preferably 0.005% or less.

Al≦0.5%

Al is a ferrite-stabilizing element that significantly raises the Ac3point in annealing and thus inhibits transformation ofnon-recrystallized ferrite to austenite, thereby interfering with thegrowth in an orientation in which the Young's modulus is advantageouslyimproved when ferrite is generated from austenite during cooling. Thus,the Al content is 0.5% or less, preferably 0.1% or less. On the otherhand, since Al is useful as a deoxidation element of steel, the Alcontent is preferably 0.01% or more.

N≦0.01%

High N content brings about slab cracking during hot rolling and maycause surface defects. Thus, N content should be 0.01% or less.

0.02%≦Ti≦0.20%

Ti is the most important element. That is, Ti inhibits recrystallizationof worked ferrite during a heating step in an annealing process so thattransformation of non-recrystallized ferrite to austenite isfacilitated, while allowing growth of ferrite, which is generated duringa cooling step after annealing, in an orientation in which the Young'smodulus is advantageously improved. In addition, fine precipitates of Ticontribute to enhancement of strength and, furthermore, have anadvantageous effect on refinement of ferrite and martensite. To obtainthis effect, the Ti content should be 0.02% or more, preferably 0.04% ormore.

On the other hand, addition of a large amount of Ti results in not allof carbonitrides being dissolved during reheating in a normal hotrolling process and coarse carbonitrides being left, thereby impedingrather than improving the effects of enhancing strength and inhibitingrecrystallization. In addition, even if hot rolling is initiateddirectly after continuous casting of a slab without subjecting the slabto cooling and subsequent reheating after the continuous casting, theamount of Ti added exceeding 0.20% only makes a small contribution tothe effects of enhancing strength and inhibiting recrystallization, andfurthermore, leads to an increase in alloy cost. Thus, Ti content shouldbe 0.20% or less.

While the basic elements have been described, it is not sufficient toonly satisfy the above-described basic elements. Rather, regarding thecontents of C, N, S and Ti, it is also necessary to satisfy thefollowing Formulae (1) and (2):

0.05≦[% C]−(12/47.9)×[% Ti*]≦0.10   (1),

where

Ti*=[%Ti]−(47.9/14)×[%N]−(47.9/32.1)×[%S]  (2), and

[% M] indicates the content (mass %) of M element.

The above relationships define the amount of C that is not fixed ascarbide. However, if a large amount of C that is not fixed as carbide ispresent exceeding 0.10%, the volume fraction of martensite increases andthe Young's modulus decreases and, furthermore, ductility deteriorates.Thus, the amount of C not fixed as carbide, as calculated by Formula(1), should be not more than 0.10%, preferably not more than 0.09%.However, if the amount of C that is not fixed as carbide is as small asless than 0.05%, then the amount of C in austenite decreases duringannealing in a two-phase region after cold rolling and, furthermore,there will be a reduced amount of martensite phase generated aftercooling, which makes it difficult to enhance strength to 780 MPa orhigher. Thus, the amount of C not fixed as carbide should be not lessthan 0.05%, preferably not less than 0.06%.

Our steel sheets may also contain the following elements as appropriate.

0.02%≦Nb≦0.10%

Similar to Ti, Nb is also an important element. Nb inhibitsrecrystallization of worked ferrite during a heating step in anannealing process after cold rolling so that transformation ofnon-recrystallized ferrite to austenite is facilitated and coarsening ofaustenite grains is inhibited, while allowing growth of ferritegenerated during a cooling step after annealing soaking, in anorientation in which Young's modulus is advantageously improved.Further, fine carbonitrides of Nb effectively contribute to enhancementof strength and, furthermore, have an advantageous effect on refinementof ferrite and martensite. To obtain this effect, the Nb content ispreferably 0.02% or more.

However, addition of a large amount of Nb results in not all ofcarbonitrides being dissolved during reheating in a normal hot rollingprocess and coarse carbonitrides being left, thereby blocking theeffects of inhibiting recrystallization of worked austenite in a hotrolling process and inhibiting recrystallization of worked ferrite in anannealing process after cold rolling. In addition, even if hot rollingis initiated directly after continuous casting of a slab withoutsubjecting the slab to cooling and subsequent reheating after continuouscasting, the amount of Nb added exceeding 0.10% only makes a smallcontribution to the effect of inhibiting recrystallization and,furthermore, leads to an increase in alloy cost. Thus, the Nb content ispreferably not more than 0.10%, more preferably not more than 0.08%.

In addition, if Nb is also contained along with Ti, a relationship ofFormula (3) below, in place of Formula (1) above, is satisfied:

0.05 [% C]−(12/92.9)×[% Nb]−(12/47.9)×[% Ti*]23 0.10   (3).

Nb forms carbide to reduce the amount of C not fixed as carbide.Accordingly, to control the amount of C not fixed as carbide to 0.05% to0.10%, if Nb is added, the value of ([% C]−(12/92.9)×[% Nb]−(12/47.9)×[%Ti*]) is controlled to 0.05% to 0.10%, preferably 0.06% to 0.09%.

0.1%≦Cr≦1.0%

Cr is an element that inhibits formation of cementite, thereby improvingquench hardenability. Cr has an effect of greatly facilitating formationof martensite phase during a cooling step after soaking in an annealingprocess. To obtain this effect, the Cr content is preferably 0.1% ormore. However, if a large amount of Cr is added, the effect attained bythe addition will be saturated and alloy cost will also increase. Thus,Cr is preferably added in an amount of 1.0% or less. In addition, when asteel sheet is used as a hot-dip galvanized steel sheet, oxides of Crformed on a surface of the steel sheet induces absence of zinc coating.Thus, the Cr content is preferably 0.5% or less.

0.1%≦Ni≦1.0%

Ni is an element that improves quench hardenability and may facilitateformation of martensite phase during a cooling step after soaking in anannealing process. In addition, Ni effectively contributes toenhancement of the strength of steel as a solid-solution-strengtheningelement. Further, in the case of Cu-added steel, surface defects areinduced during hot rolling due to cracking associated with a reductionin hot ductility. However, it is possible to inhibit occurrences of suchsurface defects by containing Ni in combination with Cu. To obtain thiseffect, the Ni content is preferably 0.1% or more. However, addition ofa large amount of Ni interferes with formation of ferrite, which isnecessary to increase the Young's modulus, during a cooling step aftersoaking and, furthermore, results in an increase in alloy cost. Thus,the Ni content is preferably 1.0% or less.

0.1%≦Mo≦1.0%

Mo is an element that improves quench hardenability and may facilitateformation of martensite phase during a cooling step after soaking in anannealing process, thereby contributing to enhancement of strength. Toobtain this effect, the Mo content is preferably 0.1% or more. However,if a large amount of Mo is added, the effect attained by the additionwill be saturated at some point and alloy cost will also increase. Thus,the Mo content is preferably 1.0% or less, more preferably 0.5% or less.

0.1%≦Cu≦2.0%

Cu is an element that improves quench hardenability and facilitatesformation of martensite phase during a cooling step after soaking in anannealing process, thereby contributing to enhancement of strength. Toobtain this effect, the Cu content is preferably 0.1% or more. However,excessive addition of Cu deteriorates hot ductility and induces surfacedefects associated with cracking during hot rolling. Thus, the Cucontent is preferably 2.0% or less.

0.0005%≦B≦0.0030%

B is an element that improves quench hardenability by inhibitingtransformation from austenite to ferrite and facilitates formation ofmartensite during a cooling step after soaking in an annealing process,thereby contributing to enhancement of strength.

To obtain this effect, the B content is preferably 0.0005% or more.However, excessive addition of B severely interferes with formation offerrite during cooling after soaking and reduces Young's modulus. Thus,the B content is preferably 0.0030% or less.

Reasons for limitations on the microstructure will now be describedbelow.

The steel sheets have a microstructure in which ferrite phase is theprimary phase, including, in area ratio, 60% or more of ferrite phaseand 15% to 35% of martensite phase.

The area ratio of ferrite phase should be 60% or more since ferritephase is effective in causing growth of texture which is advantageous inimproving the Young's modulus. In addition, since strength as well asstrength-elongation balance improve by containing martensite phase, thearea ratio of martensite phase should be 15% or more. However, if thearea ratio of martensite phase exceeds 35%, it is not possible to ensureappropriate Young's modulus in the transverse direction. Thus, the arearatio of martensite phase should be not more than 35%. Further, toimprove strength-elongation balance, a total of the area ratios offerrite phase and martensite phase should be 95% or more.

Phases other than the ferrite phase and the martensite phase may includepearlite, bainite and cementite phases, which are not problematic ifcontained in an amount of not more than 5%, preferably not more than 3%,more preferably not more than 1%.

In addition, an average grain size of ferrite exceeding 4.0 μm leads toa reduction in strength, which necessitates increasing the volumefraction of martensite phase and adding more elements, and results in adecrease in the Young's modulus and an increase in manufacturing cost.Thus, average grain size of ferrite should be 4.0 μm or less.Particularly, to satisfy a tensile strength of 780 MPa or higher in astable manner, the average grain size of ferrite is preferably 3.5 μm orless.

Moreover, an average grain size of martensite exceeding 1.5 μm increasesthe potential of progress in void linking upon working/deformation,which results in a reduction in ductility of the steel sheet. Thus, theaverage grain size of martensite should be not more than 1.5 μm, morepreferably not more than 1.0 μm.

Area ratios of ferrite phase and martensite phase were determined bysubjecting a cross-section of the steel sheet to nital etching,observing the cross-section with scanning electron microscope (SEM),taking three images of 25 μm×30 μm regions, analyzing these images byimage processing and measuring the areas of ferrite phase and martensitephase. In addition, based on the SEM images, the average grain size wascalculated by dividing a total of respective areas of ferrite phase andmartensite phase within the field of view by the number of grains inthese phases to determine an average area of the grains, the value ofwhich average area is then raised to the power of ½.

With the above-described chemical composition and microstructure, it ispossible to obtain a high-strength thin steel sheet having excellentrigidity that has a tensile strength (TS) of 780 MPa or higher in thetransverse direction, a Young's modulus of 240 GPa or higher in thetransverse direction and a strength-elongation balance (TS×El) of 16500MPa·% or more in the transverse direction.

A preferred method of manufacturing our steel sheets will now bedescribed below.

In manufacturing the steel sheets, steel having a chemical compositionin accordance with the above-described composition is prepared bysteelmaking, depending on the target strength level. Any appropriatesteelmaking process may be applied such as normal converter steelmakingprocess or electric furnace steelmaking process. The steel prepared bysteelmaking is cast into a slab, which in turn is directly subjected tohot rolling, or alternatively subjected to cooling and subsequentheating before hot rolling, under a condition of finisher deliverytemperature of 850° C. to 950° C. to obtain a hot-rolled sheet. Then,the sheet is subjected to coiling at 650° C. or lower, followed bypickling and subsequent cold rolling at a rolling reduction ratio of 60%or more. Thereafter, in an annealing step, the sheet is heated at anaverage heating rate of 15° C./s or higher within a temperature range of(Ac1-100° C.) to Ac1, held at a soaking temperature of 780° C. to 880°C. for a duration of 150 seconds or less, and then cooled to 350° C. orlower at an average cooling rate until at least 350° C. of 5° C./s to50° C./s.

In the following, reasons for the above-described limitations on themanufacturing conditions will be described.

Finisher Delivery Temperature: 850° C. to 950° C.

By controlling finisher delivery temperature to 950° C. or lower,transformation from non-recrystallized austenite to ferrite advances toprovide fine ferrite phase and, furthermore, the degree of accumulationof the crystal grains in the (112)[1-10] orientation may be increasedthrough cold rolling and annealing. However, if the finisher deliverytemperature is below 850° C., it may more likely fall below the Ar3transformation point, which results in mixing hot-rolled phase withworked phase, thereby disturbing accumulation in the (112)[1-10]orientation after the cold rolling and annealing. This also posesdifficulty in manufacture such as a significant increase in rolling loaddue to increased transformation resistance. Thus, the finisher deliverytemperature should be 850° C. to 950° C.

Coiling Temperature: 650° C. or Lower

If the coiling temperature after the finish rolling exceeds 650° C.,carbonitrides of Ti and Nb coarsen and thus the effects of inhibitingrecrystallization of ferrite and inhibiting coarsening of austenitegrains are weakened during heating stage of the annealing process afterthe cold rolling. Thus, the coiling temperature is not higher than 650°C. On the other hand, if the coiling temperature is lower than 400° C.,many hard, low temperature transformation phases are generated, whichcauses non-uniform deformation in the subsequent cold rolling, therebydisturbing accumulation in an orientation in which the Young's modulusis advantageously improved. This results in no growth of the textureafter annealing, which makes it difficult to improve Young's modulus.Further, in view of an increase in load during cold rolling after thecoiling, the coiling temperature is preferably not lower than 400° C.

Rolling Reduction Ratio during Cold Rolling: 60% or More

After the above-described coiling, the sheet is subjected to pickling,followed by cold rolling at a rolling reduction ratio of 60% or more.This cold rolling causes accumulation in the (112)[1-10] orientation inwhich Young's modulus is effectively improved. That is, growth in the(112)[1-10] orientation is caused by cold rolling to provide moreferrite grains having the (112)[1-10] orientation even in themicrostructure after the subsequent annealing process and improveYoung's modulus. To obtain this effect, the rolling reduction ratioduring cold rolling should be 60% or more, more preferably 65% or more.However, the rolling load becomes larger with higher rolling reductionratio during cold rolling so that manufacture becomes more difficult.Thus, the upper limit of the rolling reduction ratio during cold rollingis preferably 85%.

Average Heating Rate From (Ac1-100° C.) to Ac1: 15° C./s or Higher

To improve the Young's modulus of the steel sheet after annealing, it isnecessary to inhibit, during a heating step in annealing,recrystallization of ferrite that has grown during cold rolling and hasan (112)[1-10] orientation and to cause transformation from workedferrite to austenite. To this end, average heating rate should be 15°C./s or higher.

As used herein, Ac1 is Ac1 transformation temperature determined byFormula (4) below based on the contents of C, Si, Mn, Al, Ni, Cr, Cu,Mo, Ti, Nb and B expressed in mass %:

Ac1=750.8−26.6[% C]+17.6[% Si]−11.6[% Mn]−169.4[% Al]−23.0 [% Ni]+24.1[%Cr]−22.9[% Cu]+22.5[% Mo]−5.7[% Ti]+232.6[% Nb]−894.7[% B]  (4),

where [% M] indicates the content (mass %) of M element.

Soaking Temperature: 780° C. to 880° C., Soaking Duration: 150 Secondsor Less

During soaking in the annealing process, a sufficient amount of ferritetransforms to austenite, which in turn transforms again to ferriteduring cooling. This allows growth of the texture, thereby improving theYoung's modulus. In addition, if the soaking temperature is low, rolledtextures remain and elongation decreases. Thus, the soaking temperatureshould be 780° C. or higher. However, an excessively high soakingtemperature coarsens austenite grains, which makes it difficult forferrite, which results from the retransformation during cooling afterannealing, to accumulate in the (112)[1-10] orientation. Thus, thesoaking temperature should be 880° C. or lower.

In addition, coarsening of austenite grains is also caused by holding atthis temperature range for a long duration. Thus, the soaking durationshould be 150 seconds or less. On the other hand, to prevent remainingof the rolled texture and to improve elongation, the soaking duration ispreferably 15 seconds or more.

Average Cooling Rate from Soaking Temperature to at Least 350° C.: 5°C./s to 50° C./s

In the manufacturing method, it is important to control the coolingcondition after the above-described soaking treatment.

That is, formation of ferrite during cooling after soaking allows forgrowth of texture which is advantageous in improving the Young'smodulus. Accordingly, ferrite is to be formed at an area ratio of 60% ormore during this cooling step. To this end, the upper limit of thecooling rate should be 50° C./s. On the other hand, an excessively slowcooling rate hampers formation of martensite. Thus, the cooling rateshould be not lower than 5° C./s, preferably not lower than 10° C./s.

In addition, a high cooling stop temperature causes formation of bainiteand pearlite instead of martensite, which leads to a reduction instrength and an increase in the YS/TS ratio. Alternatively, even ifmartensite is formed, the hardness of martensite is reduced by temperingduring cooling and thus the contribution to enhancement of strengthbecomes small, which hampers provision of a good TS-El balance. Thus, itis necessary to conduct cooling at a predetermined cooling rate until atleast 350° C. Further, for a better TS-El balance, it is preferable toconduct cooling at a predetermined cooling rate until at least 300° C.

Thereafter, the steel sheet may be subjected to the process where thesteel sheet is passed through an overaging zone. In addition, ifmanufactured as a hot-dip galvanized steel sheet, the steel sheet may bepassed through molten zinc, or alternatively, when manufactured as ahot-dip galvannealed steel sheet, the steel sheet may be subjected to analloying process.

It should be noted that the steel sheet may be subjected to temperrolling to adjust the shape of the steel sheet, in which case there isno significant change in Young's modulus or tensile properties if thepercent elongation is not more than 0.8%, preferably not more than 0.6%.

EXAMPLES

Examples will now be described below. It should be noted that thisdisclosure is not intended to be limited to the disclosed examples.

Example 1

At first, Steel A having a chemical composition as shown in Table 1 wasprepared by steelmaking in a vacuum melting furnace. Then, Steel A wassubjected to hot rolling, pickling, cold rolling and subsequentannealing to produce a cold-rolled steel sheet. In this case, thefollowing basic conditions were set—heating condition prior to hotrolling: 1250° C. for one hour; finisher delivery temperature of hotrolling: 880° C.; sheet thickness after hot rolling: 4.4 mm; coilingcondition: process corresponding to coiling where furnace cooling wasconducted after a holding time of one hour at 600° C.; rolling reductionratio during cold rolling: 68%, sheet thickness after cold rolling: 1.4mm, average heating rate from (Ac1-100° C.) to Ac1: 20° C./s, durationat soaking temperature of 830° C.: 60 seconds, average cooling rateuntil 300° C.: 15° C./s, and subsequent cooling to room temperature: aircooling. These basic conditions are shown in Table 2.

Further, among these basic conditions, rolling reduction ratio duringcold rolling, heating rate from (Ac1-100° C.) to Ac1, soakingtemperature, quench stop temperature and cooling rate to quench stoptemperature during the annealing process were changed as shown in Table3.

After the above-described annealing, test specimens of 10 mm×50 mm werecut from the steel sheets in a direction perpendicular to the rollingdirection of the steel sheets. Then, a resonance frequency measuringdevice of lateral vibration type was used to measure the Young's modulus(Ec) in accordance with the standard (C1259) of American Society toTesting Materials. In addition, JIS No. 5 tensile test specimens werecut from the cold-rolled steel sheets, which had been subjected totemper rolling with percent elongation of 0.5%, in a directionperpendicular to the rolling direction for measuring their tensileproperties (tensile strength TS and elongation El).

It should be noted that the area ratio of ferrite phase (a) and the arearatio of martensite phase (M), as well as the average crystal grain sizeof each phase were determined by the above-mentioned method.

The obtained results are shown in Table 2 and Table 3.

TABLE 1 Steel Chemical Composition (mass %) C* Ac₁ ID C Si Mn P S Al NTi Nb (mass %) (° C.) Remarks A 0.12 1.01 2.01 0.014 0.002 0.04 0.0040.12 0.03 0.090 741.6 Conforming Steel C*: Amount of C not fixed ascarbide (C* = [% C] − (12/92.9) × [% Nb] − (12/47.9) × [% Ti*]) WhereTi* = [% Ti] − (47.9/14) × [% N] − (47.9/32.1) × [% S].

TABLE 2 Hot Rolling Cold Rolling Condition Condition Annealing ConditionFinisher Rolling Heating Rate Quench Cooling Rate to Delivery CoilingReduction Sheet from (Ac₁-100° Soaking Soaking Stop Quench Stop SteelSteel Temp. Temp. Ratio Thickness C.) to Ac₁ Temp. Duration Temp. Temp.ID Sheet (° C.) (° C.) (%) (mm) (° C./s) (° C.) (sec.) (° C.) (° C./s) AA1 880 600 68 1.41 20 830 60 300 15 Material Microstructure FerriteMartensite Ferrite Martensite Grain Grain Steel Steel Fraction FractionBalance Size Size ID Sheet (%) (%) (%) (μm) (μm) A A1 67 33 0 2.9 0.8Material Property Steel Steel YS TS El Ec TS × El ID Sheet (MPa) (MPa)(%) (GPa) (MPa · %) Remarks A A1 732 1064 16.3 252 17343 InventiveExample

TABLE 3 Hot Rolling Cold Rolling Condition Condition Annealing ConditionFinisher Rolling Heating Rate Quench Cooling Rate to Delivery CoilingReduction Sheet from (Ac₁-100° Soaking Soaking Stop Quench Stop SteelSteel Temp. Temp. Ratio Thickness C.) to Ac₁ Temp. Duration Temp. Temp.ID Sheet (° C.) (° C.) (%) (mm) (° C./s) (° C.) (sec.) (° C.) (° C./s) AA2 880 600 54 2.02 20 830 60 300 15 A A3 880 600 77 1.01 20 830 60 30015 A A4 880 600 80 0.88 20 830 60 300 15 A A5 880 600 68 1.41 10 830 60300 15 A A6 880 600 68 1.41 20 770 60 300 15 A A7 880 600 68 1.41 20 86060 300 15 A A8 880 600 68 1.41 20 830 60 600 15 A A9 880 600 68 1.41 20830 60 300  3 A A10 880 600 68 1.41 20 830 60 300 150  MaterialMicrostructure Ferrite Martensite Ferrite Martensite Grain Grain SteelSteel Fraction Fraction Balance Size Size ID Sheet (%) (%) (%) (μm) (μm)A A2 68 32 0 3.3 0.9 A A3 65 35 0 2.8 0.8 A A4 65 35 0 2.7 0.8 A A5 6634 0 4.2 1.7 A A6 89 11 0 3.2 1.4 A A7 66 34 0 3.2 0.9 A A8 85  0 P: 153.0 — A A9 87 13 0 4.1 2.3 A A10 53 47 0 3.6 1.7 Material Property SteelSteel YS TS El Ec TS × El ID Sheet (MPa) (MPa) (%) (GPa) (MPa · %)Remarks A A2 702 1049 18.1 237 18987 Comparative Example A A3 741 106215.8 255 16780 Inventive Example A A4 765 1075 15.4 254 16555 InventiveExample A A5 701 1036 16.6 233 17198 Comparative Example A A6 796 110512.7 235 14034 Comparative Example A A7 714 1032 17.3 252 17854Inventive Example A A8 473  689 24.5 245 16881 Comparative Example A A9520  763 23.2 254 17702 Comparative Example A A10 729 1124 12.7 23914275 Comparative Example P: Pearlite

A cold-rolled steel sheet (Steel Sheet Al), which was produced inaccordance with the basic conditions, exhibited a goodstrength-elongation balance and a high Young's modulus, as shown inTable 2, such that TS: 1064 MPa; El: 16.3%; TS×El: 17343 MPa·%; Ec: 252GPa; area ratio of ferrite: 67%; area ratio of martensite: 33%; ferritegrain size: 2.9 μm; and martensite grain size: 0.8 μm.

In addition, even if the rolling reduction ratio during cold rolling andthe annealing condition were changed, excellent properties were stillobtained in each case where these conditions fall within the scope ofour methods (Steel Sheets A3, A4 and A7), such as TS of 780 MPa orhigher, TS×El of 16500 MPa·% or higher and Ec of 240 GPa or higher.

Example 2

Furthermore, Steels B to N having chemical compositions shown in Table 4were prepared by steelmaking in a vacuum melting furnace. Then, thesesteels were subjected to hot rolling, pickling, cold rolling andannealing sequentially under the conditions shown in Table 5.

The cold-rolled steel sheets thus obtained were analyzed in the same wayas described in Example 1. The obtained results are shown in Table 5.

TABLE 4 Steel Chemical Composition (mass %) C* Ac₁ ID C Si Mn P S Al NTi Nb Ni Cr Cu Mo B (mass %) (° C.) Remarks B 0.10 0.85 2.03 0.015 0.0020.04 0.004 0.07 — — — — — — 0.087 732.4 Conforming Steel C 0.06 1.032.03 0.014 0.001 0.04 0.004 0.04 0.02 — — — — — 0.051 741.4 ConformingSteel D 0.08 1.03 2.02 0.014 0.002 0.04 0.003 0.05 0.03 — — — — — 0.067743.3 Conforming Steel E 0.10 1.04 2.04 0.017 0.001 0.04 0.003 0.11 0.03— — — — — 0.072 742.4 Conforming Steel F 0.10 1.02 2.03 0.014 0.001 0.040.003 0.07 0.03 — — — — — 0.082 742.3 Conforming Steel G 0.10 1.03 2.030.014 0.001 0.04 0.003 0.06 0.08 — — — — — 0.078 754.2 Conforming SteelH 0.15 1.02 2.01 0.021 0.002 0.04 0.003 0.10 — — — — — — 0.128 734.1Comparative Steel I 0.12 1.01 2.00 0.015 0.002 0.03 0.003 0.04 — — — — —— 0.113 736.9 Comparative Steel J 0.09 1.05 3.50 0.016 0.003 0.04 0.0030.07 0.02 — — — — — 0.074 723.8 Comparative Steel K 0.12 0.30 2.00 0.0170.003 0.04 0.003 0.12 — — — — — — 0.094 722.2 Comparative Steel L 0.091.41 1.97 0.015 0.003 0.04 0.003 0.08 — 0.10 — 0.20 — 0.0010 0.074 735.4Conforming Steel M 0.11 0.81 2.21 0.018 0.003 0.03 0.003 0.12 — — 0.20 —— — 0.084 735.5 Conforming Steel N 0.11 1.18 2.21 0.014 0.002 0.04 0.0030.12 — — — — 0.15 — 0.083 738.9 Conforming Steel C*: Amount of C notfixed as carbide without addition of Nb: C* = [% C] − (12/47.9) × [%Ti*] with addition of Nb: [% C] − (12/92.9) × [% Nb] − (12/47.9) × [%Ti*] Where Ti* = [% Ti] − (47.9/14) × [% N] − (47.9/32.1) × [% S]

TABLE 5 Hot Rolling Cold Rolling Condition Condition Annealing ConditionFinisher Rolling Heating Rate Quench Cooling Rate to Delivery CoilingReduction Sheet from (Ac₁-100° Soaking Soaking Stop Quench Stop SteelSteel Temp. Temp. Ratio Thickness C.) to Ac₁ Temp. Duration Temp. Temp.ID Sheet (° C.) (° C.) (%) (mm) (° C./s) (° C.) (sec.) (° C.) (° C./s) BB 880 600 68 1.41 20 830 100  300 15 C C 880 600 68 1.41 30 830 60 30015 D D 880 600 68 1.41 30 830 60 300 15 E E 880 600 68 1.41 20 830 60300 15 F F 880 600 68 1.41 20 830 60 300 15 G G 880 600 68 1.41 20 83060 300 15 H H 880 600 68 1.41 20 830 60 300 15 I I 880 600 68 1.41 20830 60 300 15 J J 880 600 68 1.41 20 800 60 300 15 K K 880 600 68 1.4120 800 60 300 15 L L 880 600 75 1.10 20 830 60 300 15 M M 880 600 751.10 20 830 60 300 15 N N 880 600 75 1.10 20 830 60 300 15 MaterialMicrostructure Ferrite Martensite Ferrite Martensite Grain Grain SteelSteel Fraction Fraction Balance Size Size ID Sheet (%) (%) (%) (μm) (μm)B B 67 33 0 3.3 1.0 C C 77 23 0 3.0 0.8 D D 74 26 0 3.2 0.9 E E 68 32 02.8 1.0 F F 69 31 0 2.9 0.9 G G 72 28 0 2.6 0.8 H H 61 39 0 3.3 1.3 I I63 37 0 3.7 1.4 J J 59 41 0 3.5 1.6 K K 73 27 0 3.4 1.1 L L 68 32 0 3.31.0 M M 69 31 0 3.2 0.9 N N 73 27 0 3.1 0.9 Material Property SteelSteel YS TS El Ec TS × El ID Sheet (MPa) (MPa) (%) (GPa) (MPa · %)Remarks B B 623 938 17.9 249 16790 Inventive Example C C 549 814 20.3250 16522 Inventive Example D D 593 895 18.9 248 16916 Inventive ExampleE E 680 1012  17.0 248 17172 Inventive Example F F 668 982 17.7 24717341 Inventive Example G G 637 942 18.4 249 17333 Inventive Example H H834 1132  13.2 235 14942 Comparative Example I I 774 1043  13.6 23614185 Comparative Example J J 810 1203  13.2 227 15880 ComparativeExample K K 549 776 18.2 246 14123 Comparative Example L L 680 1012 16.4 252 16597 Inventive Example M M 691 1020  16.3 251 16626 InventiveExample N N 673 993 17.0 251 16881 Inventive Example

As shown in Table 5, each of our steel sheets (Steel Sheets B to G and Lto N) exhibited excellent properties such as TS of 780 MPa or higher,TS×El of 16500 MPa·% or higher and Ec of 240 GPa or higher.

In contrast, Comparative Examples (Steel Sheets H to K) having chemicalcompositions out of an appropriate range are inferior in at least one oftensile strength (TS), strength-elongation balance (TS×El) and Young'smodulus (Ec).

INDUSTRIAL APPLICABILITY

It is possible to provide a thin steel sheet having both high strengthand high rigidity with a tensile strength of 780 MPa or higher and aYoung's modulus of 240 GPa or higher.

1-6. (canceled)
 7. A thin steel sheet having a composition including, inmass %, C: 0.06% to 0.12%, Si: 0.5% to 1.5%, Mn: 1.0% to 3.0%, P: 0.05%or less, S: 0.01% or less, Al: 0.5% or less, N: 0.01% or less, Ti: 0.02%to 0.20%, and the balance being Fe and incidental impurities, where thecomposition satisfies Formulae (1) and (2) below, wherein the steelsheet has a microstructure comprising a ferrite phase at an area ratioof 60% or more and a martensite phase at an area ratio of 15% to 35%,where a total of the ferrite phase and the martensite phase is 95% ormore, an average grain size of ferrite is 4.0 μm or less and an averagegrain size of martensite is 1.5 μm or less, wherein the steel sheet hasa tensile strength (TS) of 780 MPa or higher in a transverse directionperpendicular to the rolling direction, a Young's modulus of 240 GPa orhigher in the transverse direction, and a strength-elongation balance(TS×El) of 16500 MPa·% or more in the transverse direction, thestrength-elongation balance being expressed by a product of the tensilestrength (TS) and total elongation (El),0.05≦[% C]−(12/47.9)×[% Ti*]≦0.10   (1),whereTi*=[% Ti]−(47.9/14)×[% N]−(47.9/32.1)×[% S]  (2), and [% M] indicatesthe content (mass %) of M element.
 8. The thin steel sheet according toclaim 7, wherein the composition of the steel sheet further comprises,in mass %, Nb: 0.02% to 0.10%, and satisfies Formula (3) below in placeof the Formula (1):0.05 [% C]−(12/92.9)×[% Nb]−(12/47.9)×[% Ti*]23 0.10   (3).
 9. The thinsteel sheet according to claim 7, wherein the composition of the steelsheet further comprises, in mass %, one or more elements selected fromCr: 0.1% to 1.0%, Ni: 0.1% to 1.0%, Mo: 0.1% to 1.0%, Cu: 0.1% to 2.0%and B: 0.0005% to 0.0030%.
 10. A method of manufacturing a thin steelsheet comprising: in a hot rolling process, subjecting a steel materialto finish rolling and completing the finish rolling at 850° C. to 950°C. to obtain a hot-rolled steel sheet, the steel material having acomposition including, in mass %, C: 0.06% to 0.12%, Si: 0.5% to 1.5%,Mn: 1.0% to 3.0%, P: 0.05% or less, S: 0.01% or less, Al: 0.5% or less,N: 0.01% or less, Ti: 0.02% to 0.20%, and the balance being Fe andincidental impurities, where the contents of C, N, S and Ti satisfyFormulae (1) and (2): then coiling the steel sheet at 650° C. or lower;subjecting the steel sheet to pickling; and then subjecting the steelsheet to cold rolling at a rolling reduction ratio of 60% or more; in asubsequent annealing process, heating the steel sheet to a soakingtemperature of 780 to 880° C. at an average heating rate from (Ac₁-100°C.) to Ac₁ of 15° C./s or higher; holding the steel sheet at the soakingtemperature for 150 seconds or less; and cooling the steel sheet to 350°C. or lower at an average cooling rate until at least 350° C. of 5° C./sto 50° C./s,
 11. The method according to claim 10, wherein thecomposition of the steel material further comprises, in mass %, Nb:0.02% to 0.10%, and satisfies Formula (3) below in place of the Formula(1):0.05 [% C]−(12/92.9)×[% Nb]−(12/47.9)×[% Ti*]23 0.10   (3).
 12. Themethod according to claim 10, wherein the composition of the steelmaterial further comprises, in mass %, one or more elements selectedfrom Cr: 0.1% to 1.0%, Ni: 0.1% to 1.0%, Mo: 0.1% to 1.0%, Cu: 0.1% to2.0% and B: 0.0005% to 0.0030%.
 13. The thin steel sheet according toclaim 8, wherein the composition of the steel sheet further comprises,in mass %, one or more elements selected from Cr: 0.1% to 1.0%, Ni: 0.1%to 1.0%, Mo: 0.1% to 1.0%, Cu: 0.1% to 2.0% and B: 0.0005% to 0.0030%.14. The method according to claim 11, wherein the composition of thesteel material further comprises, in mass %, one or more elementsselected from Cr: 0.1% to 1.0%, Ni: 0.1% to 1.0%, Mo: 0.1% to 1.0%, Cu:0.1% to 2.0% and B: 0.0005% to 0.0030%.